Various non-aqueous alkali-metal electrochemical cells such as lithium cells are known in the art. Lithium cells, such as the one described in U.S. Pat. No. 4,828,834 to Nagaura et al. typically comprise a highly electroactive anode in the form of metallic lithium, a lithium salt dissolved in one or more organic solvents as the electrolyte, an electrochemically active cathode, which may be a chalcogenide of a transition metal or a metal oxide and a semipermeable separator placed between the anode and the cathode. The separator acts as a mechanical barrier against short-circuiting.
One drawback of lithium cells is that since metallic lithium is a highly reactive metal, it readily reacts with water vapor in air, and thus, lithium anodes must be manufactured in an entirely dry atmosphere.
Another drawback of lithium cells is the formation of lithium dendrites during the charge/discharge cycle. Lithium dendrites react with the electrolyte to form electrochemically non-active species which do not participate in subsequent charge/discharge cycles. This leads to lower discharge efficiency. Further, lithium dendrites can bridge the gap between the cathode and the anode and therefore cause cell failure via internal short-circuiting.
One way which partially overcomes low cell efficiency resulting from lithium dendrites formation is to resort to a large excess of lithium in the cell, typically a four fold excess. Lithium excess in the cell increases the thickness of the anode which is typically 150-200 microns in an AA size cell. As there exists a correlation between overall anode area and attainable power of the cell, the comparatively thick metallic lithium leads to a relatively low surface area and therefore to relatively low power density for the charge and for the discharge cycle. In addition, lithium excess in the cell reduces overall cell capacity, the larger lithium quantity is inherently more dangerous, and as lithium is comparatively expensive, cell cost is increased.
One approach to decrease dendrite formation is mechanical packaging under high pressure described in U.S. Pat. No. 4,902,589 to Dahn et al. A different approach is to charge the cell at relatively low current density, typically 0.3 mA/cm.sup.2, which results in long charging times.
A different type of rechargeable lithium cells such as the one described in PCT Application PCT/CA90/00127 to Fong et al. include an anode comprising a suitable carbon form, such as coke or graphite, intercalated with lithium ions to form Li.sub.x C.sub.6, where x is lower than 1.
A drawback of lithium cells based on anodes made of lithium intercalated with carbon is that the maximum capacity derivable from such a cell is about 377 mAh/g which is considerably lower than the theoretical value of 3860 mAh/g for pure lithium metal. AA cells with a graphite intercalated anode such as the one described in U.S. Pat. No. 4,423,125 to Samar have a maximum capacity of the order of 500 mAh compared with 700 mAh for a cell with a lithium anode.